1. Field of the Invention
The present invention is directed to a method and an apparatus for assessing pulmonary stress in a respirating subject.
2. Description of the Prior Art
In U.S. Pat. No. 4,351,344 a method and apparatus for monitoring lung compliance is disclosed. A constant flow of gas is supplied during inspiration and a pressure versus time relationship is recorded. The pressure-time relationship is analyzed with respect to linearity. More specifically, the temporal length of a linear slope segment in the pressure-time relationship is determined. The temporal length can be compared with limits and an indication of the compliance status for the patient can be made based on the comparison.
The information thus obtained is, however, insufficient and inconclusive for being properly used in determinations of the status of the lung and as a tool for improving treatment of a lung.
Mechanical ventilation is used as a life saving treatment in many circumstances. But it can also aggravate pre-existing disease and even induce lung injury if the dynamics and physiology of mechanical breath delivery are not considered.
The lung has an inherent tendency to collapse. During normal breathing this tendency is counteracted by the chest wall and a natural substance called surfactant.
In the case of disease, the collapsing tendency becomes more pronounced, giving rise to areas (alveolar units) that collapse early during exhalation/expiration and open late during inhalation/inspiration. This cyclic opening and closing of airways may initiate lung injury manifested as gross air leaks, diffuse alveolar damage, pulmonary edema and pulmonary inflammation, all of which have been termed Ventilator Induced Lung Injury (VILI). The cyclical opening and closing of alveolar units can be counteracted by the administration of a correctly set Positive End Expiratory Pressure (PEEP).
A second postulated mechanism for VILI is the delivery of large tidal volumes (which can cause volutrauma) or high end inspiratory airway pressure (which can cause barotrauma). Both may over-stretch lung tissues, leading to fluid accumulation, inflammation and increased stiffness of the lung. Baro-/volutrauma can be avoided by setting a proper tidal volume or peak pressure.
If the ventilator settings are not optimized, the period before VILI can be considered as a period of increased stress. Hence, a determination of the degree of lung stress that may follow from a specific ventilator setting can be considered as a pulmonary stress index (PSI).
It is an object of the invention to provide a method for assessing the pulmonary stress induced by the ventilatory settings for any particular patient, in particular using pulmonary stress index (PSI). PSI can be an important tool in the course of diagnosing the condition of a lung and also for determining a proper treatment causing a minimum of harm to a subject.
It is another object of the invention to provide a breathing apparatus that can perform assessments of pulmonary stress and PSI.
It is a further object of the invention to provide a breathing apparatus that can be used for obtaining a more beneficial treatment of subjects with a minimum of VILI.
This object is achieved in the inventive method by obtaining a pressure-time relationship based on a gas flow supplied to the lungs of a subject, and analyzing the profile (curve shape) of the relationship. By analyzing the profile of the relationship, important information can be extracted. In particular, determining convexity or concavity of the profile provides relevant information.
One advantageous analysis is to adopt the profile to a power equation, e.g in the form of Pao=a*tb+c, where Pao is pressure, t is time and a, b and c are constants. Determination of the constant b is particularly interesting since b is a determinant of the shape of the profile. If b equals 1, the profile consists of a straight line, if b is less than 1 the profile is concave and if b is higher than 1 the profile is convex.
Convex profiles have been found to correspond to risks of progressive over-distension of lungs (decreasing compliance) and concave profiles have been found to correspond to risks associated with cyclic closing and opening of alveolar units (increasing compliance). Profiles can also be sigmoidal, i.e., include both concave and convex portions.
Analysis can be performed on pressure-time relationship on a breath-by-breath basis or on averaged values over a number of breaths.
Another advantageous analysis to adopt the profile to a polynomial equation, e.g. in the form of Pao=xcex1+xcex2*t+xcex3*t2, where Pao is pressure, t is time and xcex1, xcex2 and xcex3 are constants. Determination of the constant xcex3 is particularly interesting since xcex3 is a determinant of the shape of the profile. If xcex3 equals O, the profile consists of a straight line, if xcex3 is less than 0 the profile is concave and if xcex3 is higher than 0 the profile is convex.
An inventive breathing apparatus that achieves the above objects has a gas regulator for regulating respiratory gas flows, a pressure gauge for (directly or indirectly) measuring a pressure, preferably the airway pressure and a control unit for controlling the gas regulator to at least supply a constant flow of a respiratory gas during inspiration phases while measuring pressure in relation to time. The control unit is further adapted to perform the methods described above.
In one preferred embodiment, the control unit is adapted to compare the constant b with an interval, preferably with a lower limit between 0.5 and 0.95 and an upper limit between 1.05 and 1.5. As long as the constant b falls within the interval, there is no pulmonary stress. If the constant b falls outside the interval there is pulmonary stress. The value of the constant b thus provides both an indication of the presence of pulmonary stress and the magnitude of it. The constant b can therefore be used as a value for pulmonary stress index, PSI.
Similar results are obtained when the constant xcex3 is used.
In another preferred embodiment, the apparatus has a display unit and an alarm unit. The control unit is further adapted to perform at least one out of a plurality of actions depending on e.g. the value of the constant b or xcex3 (pulmonary stress index). It can generate an alarm when the stress index is too high or too low, indicating that a possibly injurious therapy is being delivered to a subject. It can display the stress index, as well as the P-t relationship, on the display unit. It can calculate suitable changes in control parameters for reducing pulmonary stress and display these as options for an operator on the display unit. It can automatically re-set the control parameters in accordance with calculations of suitable changes in the control parameters. It can determine if recruiting maneuvers should be provided, and can recommend/automatically perform recruiting maneuvers, etc.
The apparatus according to the invention can advantageously be used for automatic re-setting of PEEP, tidal volume, airway pressure, I:E ratio otherventilator-controlled parameters.
A further version of the inventive breathing apparatus that achieves the above objects has at least a first gas regulator and a second gas regulator for regulating a respiratory gas flow, at least one pressure gauge and a control unit for controlling the gas regulator based on set control parameters and for establishing a pressure-time(P-t) relationship.
To achieve a proper treatment, with a minimum of stress to the lungs, the control unit is adapted to perform a series of actions, corresponding to phases.
During the first phase, the control unit controls the first gas regulator and second gas regulator to provide respiration cycles having an initial tidal volume, an initial respiratory rate, an initial inspiratory time in relation to total respiration cycle time, a constant inspiratory gas flow, an initial oxygen fraction and an initial PEEP value. All the initial values can be predefined within the control unit, calculated by the control unit based on patient information, e.g. weight, diagnosis, etc., or entered via an operator interface by an operator.
During the second phase, the control unit controls the first gas regulator and second gas regulator to provide a progressive increase of the initial PEEP value and to determine the stress index. The stress index (e.g. constant b or g) is compared with a predefined interval and when the stress index falls within the predefined interval, the control unit can proceed to the next phase.
During the third phase, the control unit controls the first gas regulator and second gas regulator to provide at least one recruiting manoeuvre having a specific inflation pressure and inflation time. The inflation pressure could be up to levels of 30-40 cmH2O or even higher if necessary. The stress index is determined and compared with the predefined interval. If the stress index falls below the predefined interval, the control unit controls the apparatus to provide a further progressive increase of the PEEP value and repeat the recruiting maneuver(s). This is continued, until the stress index exceeds the predefined interval. Thereafter, the next phase can be commenced.
During the fourth phase, the control unit continues to determine the stress index and compare it with the predefined interval. At the same time it controls the first gas regulator and the second gas regulator to provide a progressive decrease in the PEEP value. This is continued until the stress index falls within the predefined interval.
The result of all these phases is that a setting for the apparatus is achieved that does not stress the patient. Through the operator interface, an operator can repeat the entire sequence or specific phases. It can also be beneficial if the onset of some phases requires initialisation from the operator, whereby the control unit can be adapted to display requests for proceeding on the operator interface.